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I bought a new groupset and I tried installing it all myself.

Somewhere I must have messed up, because I could not get it to shift precisely. I took the bike to my local bike mechanic. He told me, he had to replace the cable and told me why but I didn't understand clearly and I was in a rush.

Now I am back home and I would love to understand. He told me: The cable I used was not correct, it was too "springy" it was something about a helix cable, but I am not too sure. Do you have an idea what he was talking about?

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Bike cable housing come in two varieties, one of them is used for brakes and has an internal wound metal structure, usually, with an inner liner to reduce friction. If you'd strip the housing from its outermost layer, you'd find an spring like metal tube. You can stretch it and it would look like a tiny notebook binding spring.

The other variety, used for shifters has an structure comprised of many straight metal wires. These wires run parallel to the whole housing direction. It also has the liner inside. (I've never found shifter housing without it).

I'ts somewhat easy to confuse one for the other, as sometimes they can look almost the same but brake cable housing is a little bit larger in diameter, both externally and internally, as the corresponding cables are different too, the brake cable being thicker.

The brake housing structure can resist the higher compression force needed, but can change its effective length a little bit when it is bent, so, when you turn the handlebar to one side or the other, they effectively change length a tiny bit, which does not affect braking, but it is enough to throw a derailleur out of the precise alignment needed to keep the chain in the cog.

Something to have in mind is to also use the correct ferrules to terminate the shifter housing ends. The ferrules are necessary to keep the housing reinforcing wires from protruding out when subjected to pressure, particularly where the housing meets frame cable stops. I've have forgot to install ferrules os mistakenly used the wrong type, and precisely what happened is that the housing wires protruded from the sheath and jammed with the main cable in the frame stop hole.

Using the correct housing and proper ferrules fully inserted ensures the length of the housing remain the same, which keeps cable tension "stable" and that makes for more precise and predictable shifting.

When this length changes, cable tension tightens and/or loosens, which in extreme cases may produce involuntary shifts, something referred to as "ghost shifting". In less drastic cases, it causes difficult shifting and drivetrain noise that returns even after you fine tune the derailleur.

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    To clarify, this is the outer cable, the fixed tube, and not the inner cable/wire.
    – Criggie
    Feb 25 at 17:04
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    It’s easiest to tell the two types of cable housing apart by looking at the cross section. Brake cable housing on the left, shifter housing on the right: 3.bp.blogspot.com/-mxF17cZZ1aw/WLB8v_zpHRI/AAAAAAAAROE/…
    – Michael
    Feb 25 at 18:04
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Jahaziel is correct about the cable housing. If you're talking about the inner cable, it needs to be pre-stretched to index correctly. I'd be a little surprised if you could buy a derailleur cable that's not pre-stretched these days (back in the old days before indexing, they weren't), but if it's not, the indexing points will go out of sync as the cable elongates.

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